This invention relates to an improved gradient-coil apparatus for imposing a gradient on the static magnetic field in a selected region of predetermined volume. More particularly, the invention relates to a gradient-coil apparatus suitable for use in a system for generating magnetic resonance in materials placed in the system for examination. The magnetic resonance system, in which the improved gradient-coil apparatus is used, includes a magnet for providing a static magnetic field along an axis. Gradients in the static magnetic field, in directions both transverse to and parallel with the axis of the magnet, are required in magnetic resonance systems in which control of magnetic-field intensity at various locations in a material being examined is desired.
The term "magnetic resonance system" refers to a system that employs the phenomenon known as magnetic resonance or nuclear magnetic resonance (NMR) to determine characteristics of materials placed within the system. The system generates magnetic resonance in the materials by exposure of them to a radio-frequency magnetic field having components transverse to the line or axis along which a static magnetic field is directed. For the purpose of controlling or detecting the occurrence or degree of magnetic resonance at various locations in the material to be examined, gradients must be imposed upon the static magnetic field. By selective variation of the gradients in the three dimensions within the region in which the material to be examined is located, it is possible to obtain data from, and form images of, planes or sections of the material. Programmed variation of the magnetic-field gradients may be used in the acquisition of data.
The static magnetic field of a magnetic resonance system generally is produced by a solenoid magnet having a field intensity directed along its axis (usually denominated the Z-axis). The corresponding flux density usually is one or more kilogauss (0.1 webers per m.sup.2). In planes transverse to this Z-axis, that is, in planes parallel to the X-Y plane, a gradient is caused to occur. A gradient in the longitudinal or Z-direction also is used. Prior art techniques typically have placed coils on a cylindrical surface having an axis coinciding with the axis of the magnet. The Z-gradient coils have been loops of wire positioned along the axis; the transverse-gradient coils have been rectangular in shape and usually four are used, two on each side of the cylinder. The transverse-gradient coils on opposite sides are positioned across from one another and are made to conform to the surface of the cylinder. An example of this structure is shown in U.S. Pat. No. 4,254,778 issued Mar. 10, 1981 to Clow et al and in U.S. Pat. No. 4,284,948 issued Aug. 18, 1981 to Young.
The prior-art gradient coil devices generally have proven unsatisfactory for the production of outstanding images in digital image processing of magnetic-resonance data as applied to potential human medical diagnostic applications. The gradient-coil apparatus of the present invention improves on the prior-art gradient-coil designs by providing magnetic-field gradients, imposed on a static and preferably uniform and homogeneous magnetic field, that are of a selected characteristic within a predetermined volume located within the spatial region enclosed by the magnet producing the static magnetic field. The characteristic particularly desired in magnetic-resonance imaging systems is a constant gradient in all of the orthogonal X and Y (transverse) and Z directions of the magnetic field as a function of position within the field.